Light fixtures that use light emitting diode (LED) technology for illumination are gaining in popularity. These fixtures are now employed more frequently in commercial, residential and public settings. One reason why LED-based light fixtures are becoming popular is that they generally have longer operational life and operate at much higher power efficiency. For example, LED-based light fixtures typically have an operational life of around 50 to 100 thousand hours, whereas, incandescent-based light fixtures typically have an operational life of only one to two thousand hours. Additionally, LED-based light fixtures typically have a light efficacy that is 5 to 10 times that of an incandescent light fixture.
Driving or supplying power to LED-based light fixtures, however, may need more consideration to ensure substantially constant illumination. In the past, LED-based light fixtures have been driven by constant output voltage ballasts or constant output current ballasts. However, these devices generally do not provide constant power to LED-based loads, and thus, cannot ensure constant illumination of the luminous loads.
Taking, as an example, a constant output voltage ballast, it typically employs output voltage feedback to ensure that the voltage across an LED-based load is substantially constant. However, the junction voltage of LED devices decreases as environment temperature increases. As a consequence, the current, as well as the power, supplied to the LED load increases with a rise in temperature. As the current increases, this, in turn, may create more heat, which results in even higher current delivered to the load. This, in effect, may result in a thermal runaway, which may eventually lead to a burn out of the LED-based load.
In the case of a constant output current ballast, it typically employs output current feedback to ensure that the current through the LED-based load is substantially constant. However, as discussed above, the junction voltage of LED devices decreases as environment temperature increases. This has the consequence of the output voltage, as well as the power, decreasing with a rise in temperature. In this case, the LED light output will decrease with rising temperature, which may be undesirable for many applications.
Thus, a ballast that regulates both the voltage and current for a luminous load to ensure substantially constant power delivered to the load is desirable. Other desirable attributes for such a ballast is improving the power factor (PF) associated with supplying power to the luminous load, and providing protection to the associated circuit and the load in case the environment temperature gets too high.